When scientists do enter the political arena, they must understand they are playing to different rules from those used in science and need to learn the rules of politics and the media. Unless they understand the rules and tactics of policy debate it is like them walking on to a tennis court equipped only with golf sticks.

Science and policy are different domains, with different cultures, different praxis, different language and requiring different navigation skills to ensure successful passage.

Scientists tend to assume that if we can put the facts on the table, sensible policies will emerge. Mystified disappointment, disillusionment and, increasingly, disengagement is often the result when emergent policies appear to ignore or fly in the face of what, to scientists, appears to be compelling evidence.

It is crucial to understand that policy questions are usually fundamentally different from science questions.

Science deals with questions like "what is happening?" "how does it function?", and sometimes predictive questions like "if x occurs under y conditions, then what?"

We are yet to devise a better way of knowing than the scientific method when trying to understand biophysical phenomena such as the natural world and the human body. Sure, it has limitations when dealing with complex, chaotic 'wicked problems'. People have been proposing new modes of scientific inquiry that could be more useful in such contexts for at least 20 years. But science still beats witchcraft, tea leaves, astrology and assertion of inexpert opinion (however vehement) hands down.

On the other hand, policy questions usually boil down to some version of "so what should we do?"

'Should' questions are rarely researchable via the scientific method.

Policy makers draw information from various sources. Brian Head describes three lenses of evidence-based policy, of which science is but one, alongside political judgement and the professional practices and organisational knowledge of the bureaucracy.

Science can help work out the most technically effective option, and economics can help determine the most efficient or cost-effective path. But the moment we ask what we should do, we immediately introduce values.

For questions about equity, access, distribution and fairness, there is usually no right answer. Policy makers, and especially politicians, are very concerned with distributional issues - "if we do x, who will be affected how and where, and by how much, at what cost?"

Science has an important role in framing public policy issues and setting out the evidence base that informs them. This is admittedly increasingly challenging on contested issues, especially those where powerful vested interests have a lot at stake.

On such issues, science finds itself having to defend long-accepted theories that fit available evidence extremely well, against well-funded campaigns that seek to confuse, distort, misrepresent and cherry-pick that evidence, to the extent they present any evidence at all. In frustration, some scientists respond with passionate advocacy for policy positions that they believe follow from available evidence.

That's where we get into tricky territory. Is it OK for a scientist to become an advocate? Can a scientist believe in a cause, advocate passionately for that cause and still be credible?

Many scientists would (and should) be uncomfortable with phrases like 'believe in a cause'. As Clive Hamilton argues, climate 'belief' is politics, not science. Dr David Schimel, one of the world's leading experts on global biogeochemical cycles and an IPCC convening lead author said at a recent forum:

I don't 'believe' in climate change any more than I believe in gravity. I believe in the scientific method. Anthropogenic global warming is a hypothesis, consistent with the overwhelming weight of evidence, that has yet to be disproved.

A key problem on big contested issues is that both scientists and policy makers lack systematic training in how to talk about uncertainty, especially in public.

Science advances essentially through disagreement, through the contest of theories, tested by evidence. However, the argument inherent in scientific progress and the irreducible uncertainty of complex phenomena can be misconstrued and misrepresented in policy debates as a lack of consensus and a basis for inaction.

This is compounded by a generally low level of scientific literacy in the wider community. Many people simply do not understand the scientific method. Few scientists are skilled at explaining the epistemology of their discipline - how they know what they know - in ways readily understood around the proverbial water cooler.

Scientists participate in policy debates along a spectrum: from researchers expert in their field, with a deep command of relevant empirical data; to expert analysts, consultants, interpreters and advisers; to well-informed observers and commentators; to advocates and activists. These are necessarily fuzzy distinctions, and individuals can operate in different roles on different issues, or even on the same issue at different times. The key is to be clear, for yourself and others, which hat you are wearing, especially in the public domain.

The ABC's Jonathan Holmes asserts that the distinction between analysis and advocacy is equally crucial for journalists.

When scientists cross the line into advocacy for a policy course, pushing a particular response to a 'so what should we do?' question, then in my view we do so more as citizens, voters and taxpayers than as scientists.

If the policy question is closely centred on the area of expertise of the scientist, then the opinion of that scientist about the best policy course should ideally be more influential than someone with no specialist knowledge of the issue.

But in an open democratic society, while some are more expert than others, an opinion is just that.

On 'what should we do?' questions, scientists need to realise that theirs is not the only lens through which answers will be found.

Professor Andrew Campbell is the Director of the Research Institute for the Environment and Livelihoods and Head of the School of Environment at Charles Darwin University. You can follow him on Twitter @AndrewCampbell2. View his full profile here.